1. Field of the Invention
The present invention relates to a drive device of a light emitting display panel in which a large number of self light emitting elements are arranged in a matrix pattern as display pixels, and more particularly to a drive device of a light emitting display panel in which the display pixels can be driven to be lit efficiently by improvement in the utilization efficiency of electrical power in a power supply section.
2. Description of the Related Art
Demand for a display panel which has a high definition image display function and which can realize a thin shape and low power consumption has increased due to popularity of cellular telephones, personal digital assistants (PDAS), and the like, and conventionally a liquid crystal display panel has been adopted in many products as the one which meets the needs thereof. Meanwhile, these days a display panel utilizing an organic EL element whose characteristic as being a self light emitting type display element is best used has been manufactured, and this have attracted attention as a next generation display panel in place of the conventional liquid crystal display panel. A background thereof is that by employing, in a light emitting layer of the element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which can be equal to practical use have been advanced.
The organic EL element is constructed by laminating a transparent electrode for example by ITO, a light emission functional layer formed of an organic material, and a metallic electrode one by one basically on a transparent substrate such as glass or the like. The light emission functional layer may be a single layer of an organic light emitting layer, or a double layer structure composed of an organic positive hole transport layer and an organic light emitting layer, or a triple layer structure composed of an organic positive hole transport layer, an organic light emitting layer, and an organic electron transport layer, or a multilayer structure in which an injection layer of electron or positive hole is inserted into an appropriate portion among these layers.
The organic EL element can be represented electrically by an equivalent circuit as shown in FIG. 1. That is, the organic EL element can be replaced by a structure composed of a diode component E as a light emitting component and a parasitic capacitance component Cp which is connected in parallel to this light emitting component E, and thus the organic EL element has been considered as a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as a displacement current and are accumulated. It can be considered that when the drive voltage then exceeds a determined voltage (light emission threshold voltage=Vth) peculiar to this element, current begins to flow from an electrode (anode side of the diode component E) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current.
FIG. 2 shows light emission static characteristics of such an organic EL element. According to these, the organic EL element emits light at an intensity L approximately proportional to a drive current I as shown in FIG. 2(a) and emits light while the current I flows drastically when the drive voltage V is the light emission threshold voltage Vth or higher as shown by a solid line in FIG. 2(b).
In other words, when the drive voltage is the light emission threshold voltage Vth or lower, current rarely flows in the EL element, and the EL element does not emit light. Therefore, the EL element has an intensity characteristic that in a light emittable region in which the drive voltage is higher than the threshold voltage Vth, the greater the value of the voltage V applied to the EL element, the higher the light emission intensity L thereof as shown by the solid line in FIG. 2(c).
Meanwhile, it has been known that physical properties of the organic EL element change due to long-term use to cause forward voltage Vf to become greater. Thus, as shown in FIG. 2(b), the V-I characteristic of the organic EL element changes in a direction shown by the arrow (characteristic shown by the broken line) due to actual use time, and therefore the intensity characteristic is also deteriorated. The organic EL element also has a problem that variations in initial intensities occur due to for example variations in deposition at the time of film formation of this element, and thus it becomes difficult to express intensity gradation faithful to an input video signal.
Further, it has also been known that the intensity property of an organic EL element changes due to changes in the temperature roughly as shown by broken lines in FIG. 2(c). That is, while the EL element has the characteristic that the greater the value of the voltage V applied thereto, the higher the light emission intensity L thereof in the light emittable region in which the drive voltage is higher than the light emission threshold voltage, the EL element also has a characteristic that the higher the temperature becomes, the lower the light emission threshold voltage becomes. Accordingly, the intensity of the EL element has a temperature dependency that the higher the temperature becomes, the lower the applied voltage by which light emission becomes possible and that the EL element is brighter at a high temperature time and is darker at a lower temperature time though the same light emittable voltage is applied.
Meanwhile, regarding the organic EL element, due to reasons that the voltage-intensity characteristic thereof is unstable with respect to temperature changes while the current-intensity characteristic thereof is stable with respect to temperature changes and that degradation of the element due to an excess current should be prevented, a constant current drive is performed in general. In this case, a drive voltage (referred to also as an output voltage) Vo which is supplied from a power supply section for example constituted by a DC/DC converter or the like to a constant current circuit has to be set, considering the following respective factors.
That is, as the factors, it is possible to enumerate the forward voltage Vf of the EL element, a variation VB of the Vf of the EL element, an aging fraction VL of the Vf, a temperature change fraction VT of the Vf, a drop voltage VD necessary for allowing the constant current circuit to perform a constant current operation, and the like. Even when these factors interact synergistically, in order to fully ensure the constant current characteristic of the constant current circuit, the drive voltage Vo has to be set at a value obtained by adding maximum values of respective voltages shown as the respective factors.
However, a case hardly occurs where the voltage value obtained by adding the maximum values of the respective voltages as described above is needed as the drive voltage Vo supplied to the constant current circuit, and in a usual state, a large power loss as a voltage drop in the constant current circuit is brought about. Therefore, this becomes a primary factor of generation of heat, thereby putting stress on organic EL elements, peripheral circuit parts, and the like.
Japanese Patent Application Laid-Open No. 2003-162255 discloses that a monitoring EL element which measures the forward voltage Vf of an EL element which is arranged in a display panel to perform light emitting display is provided other than the EL element performing light emitting display so as to control the drive voltage provided from the power supply section while utilizing the forward voltage Vf obtained from the monitoring EL element. By the structure disclosed in Japanese Patent Application Laid-Open No. 2003-162255, the drive voltage provided from the power supply section is controlled in response to aging of the EL element and changes of environmental temperature, and thus improvement in the utilization efficiency of the power supply can be expected.
In a display panel employing a self light emitting element represented by the organic EL element, a lighting ratio or an intensity (drive current) of a self light emitting element arranged in a display panel is determined by a display content (image signal), and by this a progression rate of aging of the self light emitting element is roughly determined. That is, in a case where a bright (intensity is high) image is reproduced averagely, the progression rate of an average aging of elements is advanced, and in a case where a dark (intensity is low) image is reproduced averagely, the progression rate of an average aging of the elements is retarded.
However, according to the structure disclosed in Japanese Patent Application Laid-Open No. 2003-162255, control is performed such that, in a sense, a constant current is constantly applied to the monitoring element which measures the forward voltage Vf, so that the drive voltage provided from the power supply section is controlled based on the forward voltage. Therefore, the monitoring element and the self light emitting element constituting a display panel reach a state in which their progression rates of agings become gradually different with the elapse of use time. Accordingly, even when the drive voltage provided from the power supply section is controlled while the forward voltage obtained from the monitoring element is utilized as in the structure disclosed in Japanese Patent Application Laid-Open No. 2003-162255, it becomes impossible to maintain the utilization efficiency of electrical power in the power supply section at an optimum state.
That is, since the forward voltage obtained from the monitoring element and the average forward voltage of self light emitting elements constituting a display panel have different progression rates of agings, they gradually dissociate, and it becomes impossible to constantly supply an optimum drive voltage from the power supply section in response to the advance of aging of the self light emitting elements constituting the display panel. In other words, while it is anticipated that the dissociation between the forward voltage obtained by the monitoring element and the average forward voltage of self light emitting elements constituting a display panel occurs, a higher power supply voltage has to be set initially in the power supply section. Thus, there is a problem that useless power consumption is generated in an initial stage or a standard state.